This is difficult to troubleshoot without actually having an account on this site, since submitting dummy values for the username and password naturally will result in the same error (“Invalid Email Address or Password. Please try again.”).
My best guess is that this may be related to the onchange=passCheck(); attribute in the input field.
What happens if you mouseclick into the username and/or password fields before submitting (but after autofilling)?
Never mind. Problem was that the password length was limited to 20 characters despite no wording to that effect, my password was 21 characters in Bitwarden but only the first 20 recognized by the site, and when autofilled the full 21 was put into the online box but when copied and pasted apparently only the first 20 was filled.